Conventionally varieties of devices are contrived for chairs used in an office or the like in order to improve comfort to sit. As an example, a rear portion of a face to be seated is inclined a little so as to make the rear portion thereof lifted or a bottom portion of the back support is partially projected forward.
This arrangement makes it possible to coincide an angle of inclination of the rear portion of the face to be seated with an angle of a pelvis of a person who sits on the chair and to make the lower portion of the back support in a shape that supports a lumbar portion of the person who sits on the chair, which brings comfortable and stable feeling to sit.
However, if a shape of the rear portion of the face to be seated or of the bottom portion of the back support is standardized, the pelvis or lumbar portion of the person who sits on the chair might not be supported stably due to difference of a body proportion or of a way to sit, which might cause for the person uncomfortable feeling to sit.
In order to solve the above problems the present claimed invention intends to provide a chair in which a boundary between a seat and a back can be adjusted to a most comfortable position for a person who sits on the chair so as to obtain comfortable feeling to sit in spite of difference of a body proportion or of a way to sit.